Purified molybdenum cofactor from nitrogenase and xanthine oxidase will be compared with respect to molybdenum content, iron content, and amino acid sequence of the peptide. Detailed spectral and chemical analyses should yield insight into the exact structure of the molybdenum cofactor. Uptake of molybdenum from the medium and formation of the molybdenum cofactor will be studied in Klebsiella pneumoniae and Azotobacter vinelandii. Complementation and genetic analyses will determine the number of genes involved with formation of the molybdenum cofactor in K. pneumoniae. We shall determine the function of each of the gene products. BIBLIOGRAPHIC REFERENCES: Pienkos, P.T. and W.J. Brill. 1977. Molybdenum storage in Azotobacter vinelandii. Abst. Ann. Meet. Amer. Soc. Microbiol., in press. Pienkos, P.T., V.K. Shah, and W.J. Brill. 1977. Uptake and role of molybdenum in nitrogen-fixing bacteria. Advances in Chemistry Series, in press.